Damma vs. Shadda — What's the Difference?

(linguistics) In the Arabic script, the vowel point for u, appearing as a small curl placed above a letter ( ـُ ) and designating a short u /u/. If the Arabic letter tr=wāw immediately follows, it indicates a long ū /uː/.

Shaddah (Arabic: شَدّة shaddah [ˈʃæd.dæ], "[sign of] emphasis", also called by the verbal noun from the same root, tashdid تشديد tashdīd "emphasis") is one of the diacritics used with the Arabic alphabet, indicating a geminated consonant. It is functionally equivalent to writing a consonant twice in the orthographies of languages like Latin, Italian, Swedish, and Ancient Greek, and is thus rendered in Latin script in most schemes of Arabic transliteration, e.g.

A diacritic (◌ّ) used in the Arabic script to indicate gemination of a consonant.